Usually, in mechanical watches, the energy source for driving the train and the hands, and other functions, is a spiral spring wound inside a barrel.
When a mechanical watch is not wound regularly, or when a self-winding watch is not worn for several hours, the watch spring is completely let down, and the watch stops. The average power reserve of a normal watch whose mainspring is completely wound is around 40 to 60 hours.
There are several reasons for a watch to discharge energy until it stops, especially not wearing a watch for some time (storage, weekend, etc . . . ) or forgetting to wind the watch if it does not have a self-winding mechanism. It is therefore useful to have a watch with a greater than average power reserve.
In the past, attempts have already been made to overcome the problem of power reserve. For example, EP Patent Application No 2455820 proposes the use of a drive member comprising a barrel inside which are mounted two superposed and coaxial springs.
This document simply provides an example of watches with two barrels or two mainsprings since there are many similar publications.
Such proposed solutions to overcome the problem of increasing power reserve are of little interest, because the power reserve is given priority at the expense of available space.